Materials are known for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc.
The packages are formed from a continuous web of packaging material. According to one known technique, the web is longitudinally sealed to form a continuous tube.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene, and, in the case of aseptic packages for long-storage products, such as UHT milk, also comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product.
To produce aseptic packages, the web of packaging material is unwound off a reel and fed through an aseptic chamber in which it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is later vaporized by heating and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity.
The sterilized web is then folded into a cylinder and sealed longitudinally to form, in known manner, a continuous, vertical, longitudinally sealed tube. The tube of packaging material, in other words, forms an extension of the aseptic chamber, and is filled continuously with the pourable food product and then fed to a form-and-seal unit for forming the individual packages and on which pairs of jaws grip and seal the tube transversely to form pillow packs.
The pillow packs are then separated by cutting the sealing portion between the packs, and are fed to a final folding station where they are folded mechanically into the shape of the finished packages.
The various operations in the packaging material manufacturing process are performed using, as a reference, a register mark or marks printed on the material at the first printing stage.
The register mark or marks include a printed code that is normally also used on the forming machine to control feed of the material through the various work stations. More specifically, as is known, a so-called “decoration correcting” device acts on the packages being formed to variously “draw” the material in the feed direction and ensure performance of the mechanical forming operations matches the decoration on the packages.
On known packaging machines, before the web is sealed longitudinally to form the tube, the transverse position of the web must normally also be controlled, e.g. to perform auxiliary operations, such as cutting and applying removable tabs or opening devices.
It is also necessary to control the angular position of the tube on the sealing unit, which may vary, in use, with respect to the desired angular position, on account of the lateral edges of the web not being perfectly straight, of the effect produced by the pairs of jaws successively striking the tube, and of the effects due to the web tensioning variations.
As this may have negative effects on the quality of the longitudinal and transverse seals, and on the accuracy with which the packages are formed, known machines are provided with devices for manually adjusting the angular position of the tube. Such devices, however, are relatively time-consuming, and may involve shutting down the machine with consequent loss of production. Systems have also been proposed for automatically adjusting the angular position of the tube of packaging material, but call for the use of a dedicated sensor to determine the position of the superimposed material layers at the longitudinal seal.